Sagan was one of the best-known planetary scientists and science popularizers of the twentieth century, a winner of the Pulitzer Prize and the Public Welfare Medal of the U.S. National Academy of Sciences. To the general public, he is mainly recalled as a television personality— in particular, as a suave commentator on the possibility of extraterrestrial life and on the robotic exploration of the solar system. Secondarily, and more controversially, he is recalled as a firm secularist who defended the superiority of science and reason against what he depicted as pseudo-science and irrationalism. He is also remembered as an anti–nuclear weapons activist who clashed with the United States’s right-wing presidential leadership during the last decade of the Cold War. Diverse in the extreme, Sagan attained fame or notoriety for his imaginative speculations on subjects as different as astronomy, neuro-science, biomolecular evolution, nuclear weapons policy, hypothetical exploration strategies for interstellar spaceships, and the physics of dust transport and radiative transfer in planetary atmospheres. The author or coauthor of numerous books and hundreds of scientific papers, he was an urbane, dryly witty speaker who made many appearances on television and hosted the controversial television science series Cosmos (1980). Of his popular books, one won the Pulitzer Prize; another inspired a Hollywood “blockbuster” film that envisioned radio communication with semi-godlike aliens from other worlds.

Origins Born Carl Edward Sagan in Depression-era Brooklyn, New York, he was the first of two children (and the only son) of a Jewish couple, Samuel Sagan and Rachel Molly Gruber Sagan. Samuel Sagan was a Ukrainian émigré who worked his way up from a job as a garment cutter to factory manager. Born in the United States but raised for several years in Austria, Rachel Sagan was a smart, abrasive woman who pressured her son to think logically and to succeed. Even as an adult, according to some accounts, Carl was neurotically anxious about his relationship with his mother.

When he expressed curiosity about stars at age five, his mother suggested that he go to the library and find a book on the subject. He complied, thereby launching his astronomical career. Carl’s parents took him to the New York World’s Fair of 1939–1940, where he thrilled to see its “extremely technocratic” vision of the future. As an adolescent he loved reading science-fiction stories in “pulp” books and magazines. He was thrilled by reports of “flying saucers” that, he suspected, might be extraterrestrial spaceships. Also while young he became a religious skeptic at least partly because he discovered contradictions in the Bible. (Throughout his life he seemed to waver between agnosticism and atheism. Even so, he was friendly with theologians and pondered quasi-religious themes in his only novel.)

Education and Interest in Extraterrestrial Life Carl attended high school in Rahway, New Jersey. He became president of the chemistry club and was voted the male students’ “Class Brain” and “Most Likely to Succeed.” Graduating in 1951, he entered the University of Chicago just shy of his seventeenth birthday. He imbibed the rich diet of classically based education then offered by the school’s controversial “Hutchins program.” He was a very good, although not brilliant, student in physics. Also, he ran the astronomy club’s “theoretical” section (but he cared little for observing). A budding showman, he worked at the campus radio station, where he scripted a program on spaceflight and flying saucers. Although he later became an aggressive UFO skeptic, as an undergraduate he took the “saucer” phenomenon seriously. He even wrote the U.S. Secretary of State to ask if the nation could defend itself in the “extremely remote” chance of an alien attack.

His fascination with other worlds was strengthened by a famous experiment conducted on campus during his undergraduate years. Stanley Miller, a student of Harold Urey, experimentally simulated a reducing (hydrogen-rich) atmosphere in which methane and ammonia were exposed to electrical discharges. The product was scummy layers of amino acids, the building blocks of proteins. The Miller-Urey experiment had a powerful impact on young Sagan. As a religious skeptic, he welcomed physical explanations for the origins of life, as opposed to biblical or vitalistic ones. Even more appealing to him were the extraterrestrial implications of Miller’s experiment: Because amino acids formed readily in the proper chemical environment, the building blocks of life might exist on innumerable worlds. Given enough time, they might evolve into living, even intelligent, creatures.

After a period of decline, scientific openness to the idea of extraterrestrials had slowly revived in the 1940s and 1950s. The Miller-Urey experiment was one reason, which (as noted) implied prebiotic molecules might emerge readily wherever the planetary conditions were right. Another reason was Carl Friedrich von Weizsäcker’s wartime revival and updating of the Kant-Laplace nebular hypothesis of solar system formation: In his reformulation of the theory, planets were once again seen as a common byproduct of stellar birth. Meanwhile, recent technological innovations inspired new conjectures about extraterrestrial life. For example, after the postwar construction of large radio telescopes, Giuseppe Cocconi and Philip Morrison (1959) suggested the “dishes” might detect intelligent radio signals from interstellar civilizations. The late 1950s also inaugurated the space age: At last, robotic spaceships could directly visit and explore nearby planets, to determine whether they were inhabited by anything from microbes to “advanced” creatures. (As late as 1962, Science magazine ran an article that seriously speculated about the remote possibility of intelligent Martians.)

In this atmosphere of revived interest in alien life, young Sagan felt comfortable exploring the idea. He could be arrogant and abrasive, but he was also capable of great charm, and he knew how to convert older, more powerful scientists into mentors and allies. At his mother’s behest, he obtained a summer job in 1952 as a lab aide to the geneticist Hermann J. Muller at Indiana University. A Nobel laureate, Muller was intrigued by the clever, enthusiastic young man, and even patiently listened to his youthful conjectures about UFOs. Thanks to Muller’s clout, the twenty-three-year-old Sagan was able to publish his first scientific article, “Radiation and the Origin of the Gene,” in the journal Evolution in 1957. At Chicago Sagan received an AB degree with general and special honors in 1954, an SB in physics in 1955, and a master’s degree in physics in 1956. Then, under the supervision of Gerard P. Kuiper, Sagan began work on a doctoral thesis in the department of astronomy and astrophysics at the University of Chicago’s Yerkes Observatory in Williams Bay, Wisconsin, where his distinguished instructors included Subrahmanyan Chandrasekhar. Sagan also gave public lectures at the observatory and hosted a lecture series starring notable scientists. In 1957, he married Lynn Alexander, a fellow student at the university. They had two children, Dorion (later a noted science writer) and Jeremy.

Despite “some faculty opposition” (as Kuiper called it), Sagan’s doctoral thesis was accepted in 1960. Titled “Physical Studies of Planets,” the unusually speculative dissertation explored three questions: (1) Could Miller-Urey-style processes and subsequent evolution have generated organic molecules and even microbes on the early Moon?; (2) Might complex organic molecules exist at the planet Jupiter, and if so, could their spectral signatures be detected remotely via astronomical spectroscopy?; and (3) What caused the intense microwave radiation from Venus, which was discovered by Cornell H. Mayer and others in 1956? In his dissertation, Sagan suggested that Miller-Urey-type processes might have occurred on the primordial Moon. Thus, he warned, future explorers must avoid ferrying biological contaminants to the surfaces of other worlds. Otherwise those contaminants might harm extant life forms or confuse future generations of researchers, who could have trouble distinguishing between native and imported species.

Thanks to Kuiper, Sagan was able to conduct a secret study for the U.S. Air Force–related Armour Research Foundation on the possibility and effects of detonating nuclear weapons in the Moon. (After his death, a biographer accused Sagan of violating security rules by confiding the existence of the secret study to a limited number of people in order to boost his chances of winning a fellowship in California. For details, see Davidson [1999], Reiffel [2000], and Broad [2000]).

As for the Venusian part of Sagan’s dissertation: In 1940, based on spectroscopic evidence of Abūndant carbon dioxide in the Venusian atmosphere, Rupert Wildt

argued that the planet’s atmosphere was exceedingly hot because the gas trapped infrared radiation (the greenhouse effect). Wildt calculated a high surface temperature, over 400 Kelvin, for Venus; even so, that number was on the low side if Mayer and his collaborators’ subsequent microwave observations were correct. After uncovering Wildt’s paper, Sagan modified it to account for the temperature discrepancy. He posited an extra ingredient in the Venusian atmosphere—water vapor, which, he proposed, trapped infrared radiation that would have otherwise leaked past the carbon dioxide. Nowadays Sagan’s paper is recalled as a landmark in modern Venus studies. Even so, his model seriously overestimated the Abūndance of water vapor and nitrogen in the Venusian atmosphere, severely underestimated the atmospheric density, and significantly underestimated the surface temperature by about 150 Kelvin; it is nowadays thought to be about 750 Kelvin.

The Sagans’ marriage was unhappy and they divorced after several years. His wife later remarried and became Lynn Margulis, a celebrated microbiologist. Although she retained a wry affection for her ex-husband, she recalled him after his death as a man with a “limited capacity for any kind of personal relationship. … He had to have ten thousand people all adulating him to make up for that fear that his mother was going to leave him.”

Rise to Fame In 1960, Sagan won a postdoctoral two-year Miller Fellowship at the University of California at Berkeley. During his time in California (which included a brief stint at Stanford), he helped a scientific team develop an infrared radiometer for NASA’s Mariner 2 robotic probe. The probe flew by Venus in December 1962 and measured a surface temperature of about 800 Kelvin. Also significantly, Mariner 2’s data indicated that the microwave radiation came from the planetary surface, as Sagan’s greenhouse model implied, rather than from a highly ionized region far above the planet, as skeptics had proposed.

Sagan also won another influential ally in another future Nobelist, genetics professor Joshua Lederberg, who was then at the University of Wisconsin. Sagan and Lederberg coauthored an article, “Microenvironments for Life on Mars,” in the Proceedings of the National Academy of Sciences in 1962. Lederberg treated Sagan as a kind of protégé. Thanks to the older man, Sagan gained entrée to inner sanctums of scientific influence—for example, to a scientific panel on planetary biocontamination that included some of the most distinguished researchers of the day. Among them were Melvin Calvin, Tommy Gold, Harold Urey, and Norman Horowitz, all of whom would later figure (positively or negatively) in Sagan’s career. After Sagan’s death, Lederberg recalled how he had acted as Sagan’s early protector. He regarded the younger man as a “very bright young guy, full of ideas. … Some of them [were] kooky, but they were well worth listening to. … I was often his protector and defender from folks who thought he was wild” (Davidson, p. 90).

Sagan was fascinated by the possibility of radio communication with alien civilizations many light years from Earth. For a few months in 1960, radio astronomer Frank Drake launched the first space age search for alien signals at a radio telescope in Green Bank, West Virginia. In later years, when numerous similar radio telescope searches were conducted around the world, they became collectively known as the search for extraterrestrial intelligence or SETI. In late 1961, SETI strategies were discussed at a scientific workshop in Green Bank. The attendees included Sagan, Drake, Cocconi, Morrison, Calvin, Otto Struve, and other luminaries. Sagan argued that thousands of technically advanced civilizations might be scattered across the galaxy. Some of these societies might be able to communicate with Earth via radio. The back-and-forth messaging might take centuries because of Albert Einstein’s speed limit on electromagnetic waves, but the participants felt the search for such signals was worthwhile anyway.

During Sagan’s California period, he, Cyril Ponnamperuma, and Ruth Mariner published in Nature their claim to have synthesized adenosine triphosphate (ATP), a major source of cellular energy. They appear to have innocently misinterpreted their results, though. After Sagan and Ponnamperuma died, Mariner told a Sagan biographer that the experiment had produced far less ATP (if any) than was originally reported (Davidson, pp. 157–158).

In the early 1960s, two Sagan admirers, the Harvard astronomers Donald Menzel and Fred Whipple, obtained for him joint postings as an assistant professor of astronomy at Harvard and as a staff member at the nearby Smithsonian Astrophysical Observatory. About the same time, Sagan assumed an editorial post at the planetary science journal Icarus. He served as associate editor from 1962 until 1969, after which he served for a decade as editor in chief. True to form, he saw to it that Icarus ran articles on highly unusual topics (e.g., Francis Crick and Leslie Orgel’s thesis [1973] that terrestrial life evolved from organic materials launched to Earth by alien civilizations).

Many radical young academics mellow as they age and approach tenure review. Not Sagan: In the mid-to-late 1960s he spoke out on issues such as the feasibility of interstellar flight and the chance that aliens visited Earth thousands of years ago. In 1967 he and Harold Morowitz proposed in a joint letter to Nature that creatures resembling “gas bags” float high in the atmosphere of Venus. Sagan also publicly discussed the then-headline-making topic of UFOs; he even testified to Congress on the subject. By that time he was an outspoken skeptic of the “saucers,” although he evidently was not skeptical enough for Menzel, who privately complained that Sagan expressed his skepticism in artfully convoluted sentences that others might misinterpret. Meanwhile, Urey grumbled that Sagan speculated wildly about the possible molecular constituents of other planets.

Sagan attracted early press attention by proposing to “terraform” Venus into a habitable world. Barely thirty years old, the tall, rail-thin, handsome astronomer dazzled the media with his geniality, his patience with dunderhead questions, and his crystal-clear explanations, which he peppered with jokes and evocative metaphors. Like William F. Buckley and Truman Capote, Sagan owed his public celebrity partly to his memorable style of speech, which television comedians later mimicked; his voice was deep yet honey-smooth, his delivery melodious yet eccentrically enunciated. Not everyone was charmed: Some regarded him as arrogant or rude. When Sagan served briefly as an advisor on the famous 1968 science-fiction film 2001: A Space Odyssey, his demeanor repelled the director, Stanley Kubrick, who quit communicating with him. In later years, Sagan would fight bitterly with the producer of his television series Cosmos, who also stopped talking to him.

His most enduring scientific collaboration was with his first graduate student, James B. Pollack, starting in the mid-1960s when they were both at Harvard. The brightness variations of Mars were a long-standing puzzle: What caused them? Some experts attributed them to vegetation changes. In a 1966 article for Nature, Sagan and Pollack prophetically suggested the cause was windblown dust. Ironically, they were right partly for the wrong reason: The proposal was indirectly inspired by their (probably) erroneous thesis that Mars has undergone global-scale horizontal crustal movements akin to terrestrial plate tectonics.

In the 1960s, the Soviet astrophysicist Iosif S. Shklovskii wrote a popular science book, Universe, Life, Mind. Sagan learned about the book and contacted Shklovskii to suggest that they republish it in English, along with remarks inserted by Sagan. Shklovskii agreed. As it turned out, Sagan’s inserts more than doubled the size of the text; in 1966 it was published in English as Intelligent Life in the Universe. The cover was striking: It bore no title or authors’ names, only a black-and-white photo of a galaxy with the edge of Earth in the foreground. The book was a mix of semitechnical analysis and Saganish ebullience. For example, he described the potential for interstellar flight and radio communication with aliens. He also devoted a chapter to his amusing experience in a California courtroom, where he had provided scientific testimony against an accused embezzler who claimed to have contacted flying saucer pilots.

In 1968 Sagan married the aspiring artist Linda Salz-man. They had one son, Nicholas. That same year, Harvard officials denied Sagan tenure, at least partly because Urey had written a harsh anti-Sagan letter to the university. (Urey later apologized to Sagan.) Thomas Gold, the chair of astronomy at Cornell, who was himself a noted controversial theorist, spied a kindred spirit in Sagan and offered him a job. Sagan accepted. Located in a small, quasi-bohemian town in upstate New York, Cornell was Sagan’s academic home for the rest of his life.

Sagan never ceased trying to find excuses for regarding seemingly inhabitable planets as potential abodes of life, either now or in the past. In 1972, Sagan and George Mullen theorized that early Earth had been veiled in ammonia gas, which trapped solar infrared radiation; this kept the planet warm enough for life to emerge. Likewise, Sagan reasoned that thanks to greenhouse gases, the Martian atmosphere might have once been warm enough to support the emergence of life. Sagan also believed Martian life might still exist to the present—perhaps even large animals the size of polar bears! To some scientists (Urey among them), Sagan’s speculations raised doubts about his rigor as a theorist. Some of his hypotheses were so cagily worded, so well-shielded against falsification, that they offer interesting case studies for those scholars who analyze the flightier forms of scientific rhetoric. For example, Sagan and Mullen wrote in their 1972 paper: “Earlier conditions on Mars may have been much more clement. … It is a debatable but hardly quixotic contention that martian organisms may have been able to adapt to the increasingly rigorous Martian environment and may still be present.” Some space scientists, even those who liked Sagan and respected his breadth of technical knowledge, grumbled about his undisciplined mouth. They apparently feared that his public pronouncements about Mars risked raising public hopes which might later be disappointed. (And indeed, they would be disappointed: The Vikings landed on Mars in 1976 and failed to detect evidence of life, even of organic molecules.) Perhaps not coincidentally, Sagan was secretly an enthusiastic smoker of marijuana. He argued (in an essay published under a pseudonym, “Mr. X”) that the drug enhanced his intellectual, aesthetic, and emotional powers. (His authorship of the essay was revealed after his death.)

Sagan’s later books of the 1970s became best sellers, thanks substantially to his appearances on a popular late-night television talk show hosted by Johnny Carson. The Cosmic Connection (1973) was a boyish riff on the wonders of the cosmos and the possibility of interstellar travel. The Dragons of Eden(1977) was an intellectual high-dive act, a theoretically reckless amateur’s speculations on neuroanatomy, hominid evolution, mythology, and dreams that won the Pulitzer Prize.

Scientific Celebrity Sagan’s best-publicized “scientific” stunts had little to do with theoretical or experimental science. Rather, they were flamboyant quasi-artistic gestures (akin to Christo's) guided by a liberal ethic that might be called cosmic cosmopolitanism. His Enlightenment-style faith (no doubt inspired by his youthful love of science-fiction tales in which humans and aliens interacted easily) that symbolic and linguistic systems are universally commensurable is most memorably—and controversially— represented by the Pioneer plaques. In the early 1970s, Drake, Carl Sagan, and Linda Sagan designed a plaque for the Pioneer 10 and Pioneer 11 space probes that were scheduled to visit the outer planets and (because their speed exceeded solar escape velocity) would eventually leave the solar system. Sagan argued that extraterrestrials might eventually recover the spaceships millions of years from now, though he acknowledged the probability was extremely low. The plaques were covered with various symbols that were cleverly designed to convey basic information about the spaceship’s planet of origin. The plaques included a cartoonlike drawing of a naked man and woman: The male raised his hand, as if in greeting. Skeptics (including the art historian Ernst H. Gombrich, who had long emphasized the historical and cultural contingency of aesthetic conventions) pointed out that the lines and dots on the plaque, even the human figures, might be incomprehensible to extraterrestrials. Despite Sagan’s liberal political values, other critics complained that both human figures appeared to be white skinned, and feminists demanded to know why the woman was shown deferentially standing behind the man.

The episode exposed Sagan’s naïveté. He was determined to do better on his next big stunt: the creation of a phonograph-style disk of acoustically recorded images and music from the world’s different cultures and ethnicities—a sort of acoustical version of Edward Steichen’s “Family of Man” photographic show of the early Cold War era. Sagan and several friends (including his future wife Ann Druyan) designed the recordings. They placed copies of them aboard two other twin deep-space probes, Voyager 1 and Voyager 2, which were launched in 1977.

Sagan was intrigued by astronomical observations of what appeared to be rust-red materials on the surfaces of other worlds. He wondered: Were these prebiological substances? In experiments with Bishun Khare, Sagan tried to mimic the formation of such substances in simulated planetary atmospheres. One product of this research was a brown, gunky substance that Sagan dubbed “tholins” (after the Greek word for mud). Sagan enjoyed doing back-of-the-envelope-style calculations on the odds of protobiotic molecules emerging on a given world, and his calculations were almost always highly optimistic. For example, writing in 1971 in Science magazine, he and Khare proposed that on early Earth, volcanically emitted hydrogen sulfide could have absorbed long-wavelength solar ultraviolet radiation and generated up to 200 kilograms of amino acids per square centimeter over a billion years—“a huge number suggesting congenial conditions for the origin of life.”

At a time when many scholars (e.g., Thomas Kuhn and Paul Feyerabend) were reassessing science’s traditional claims to epistemic and social authority, Sagan continued to champion a view favored by most science pedagogues—namely, that there are clear-cut logical and methodological distinctions (i.e., lines of demarcation) between science and pseudoscience. Dire social consequences might ensue if society failed to respect these distinctions, Sagan suggested. His brilliant public dismantling of the pseudo-astrophysical theories of psychoanalyst Immanuel Velikovsky destroyed the latter’s already shaky credibility. In the 1970s Sagan was a founding member of the Committee for the Scientific Investigation of Claims of the Paranormal (CSICOP), an antioccult, antipseudo-science spin-off of the American Humanist Association that published a puckish magazine, Skeptical Inquirer.

Sagan’s best-known philosophical principle is his oft-quoted remark that extraordinary claims require extraordinary evidence. His approach to philosophy of science was traditional and polemical, not searching or scholarly. Even so, he was too genial and broad-spirited to rally behind the conservative wing of the unfolding “science wars” of the 1980s and 1990s. He had a strong sense of fairness, even when dealing with cranks. Although he despised astrology, he refused to sign a petition protesting popular interest in astrology because he believed such gestures were futile and appeared repressive. In his last years he complained there was “some degree of truth” to the charge that CSICOP activists “tend to be dismissive, closed-mined [sic] and given to ad hominem arguments,” as he said in a 1993 letter to an official of the organization (unpublished letter, Sagan to Kendrick FRāzīer, 5 April 1993).

In 1980, Sagan hosted Cosmos, a highly publicized thirteen-part public-television series on science that eventually reached several hundred million viewers around the globe. The series used special effects that were highly elaborate for the time—for example, a lifelike simulation of the interior of the Library of Alexandria; through its imaginary hallways, Sagan strolled while reminiscing about its past glories (including the great female scholar Hypatia, whose cruel fate he mentioned at the urging of his feminist wife). The television show also deployed the then-novel computer technique of “morphing” to dramatize the evolution of life-forms. The series also depicted episodes in the history of science, such as Johannes Kepler’s theoretical grapplings with the enigmas of the orbit of Mars. Time magazine ran a cover story on Sagan that called him “the prince of popularizers, the nation’s scientific mentor to the masses.” A recurrent theme of the show—and of Sagan’s writings in general—was the vastness of space and time and the comparative triviality of life on Earth. All of history’s heroes and villains had pontificated and battled on that “pale blue dot,” and to what end? Many viewers were thrilled by Sagan’s musings, but some critics smirked at his pomposity while the religious fumed at his overt religious skepticism. “Why is Carl Sagan so lonely?” asked the novelist Walker Percy. He answered that to Sagan, the cosmos was nothing but “matter in interaction, [and] there is no one left to talk to except other transcending intelligences from other worlds.” Some commentators praised the show for bringing scientific ideas to vivid life; others ridiculed the beatific gaze on Sagan’s face as he pretended to pilot a cosmic spaceship.

In 1981, Sagan won a $2 million advance to write a science-fiction novel, Contact. The resulting book, published in 1985, was an alternately enthralling and sluggish tome with a central character, radio astronomer Ellie Arroway, whose life, career, and values (including her religious skepticism) resemble Sagan's. In 1997 the book was released as a major Hollywood film starring Jodie Foster.

Anti–Nuclear Weapons Activism In the late 1970s Sagan left his wife Linda for Druyan, a fledgling writer with leftist political sympathies. The couple had two children, Alexandra and Samuel. During their intellectually intense marriage, he and “Annie” coauthored books including Shadows of Forgotten Ancestors: A Search for Who We Are(1992). Inspired by his anti–nuclear weapons activism of the 1980s, Shadows mused on the social and evolutionary roots of human self-destructiveness. It is the most mature and elegantly written of his books, and occasionally flirts with experimental narrative styles. Druyan’s literary and political influence on Sagan is evident in many passages— for example, a paragraph about the nature–nurture debate: Bigots stress “the primate connection to the veldt and the ghetto, but never, ever, perish the thought, to the boardroom or the military academy or, God forbid, to the Senate chamber or the House of Lords, to Buckingham Palace or Pennsylvania Avenue” (p. 67).

Sagan’s new political outspokenness owed much to the election of Ronald Reagan as U.S. president in 1980. Reagan’s administration, critics feared, was favorably oriented to nuclear “first strikes,” that is, to preemptive nuclear attacks on the Soviet Union. In reply, Sagan and colleagues developed their famous “nuclear winter” hypothesis. Inspired partly by the early studies of Paul Crutzen and John Birks, Sagan and four other scientists— Richard Turco, Brian Toon, Thomas P. Ackerman, and Pollack (collectively known as TTAPS, with the “S” for Sagan)—calculated that even a “small” nuclear war involving hundreds of warheads might spark gigantic urban fires. The blazes could unleash vast clouds of soot that would darken the sky, blocking solar radiation. The possible result: a planetwide cooling or freeze, which could ravage agriculture and, in a worst-case scenario, might kill billions. Thus, Sagan reasoned, only a foolish nation would start a nuclear war because nuclear winter would quickly spoil any initial tactical advantage that it obtained by launching the first strike. The risk of nuclear winter could be lessened, he stressed, if the United States and Soviet Union radically shrank the size of their nuclear arsenals. (It was never clear how many bombs would have to be detonated in order to trigger a nuclear winter.)

Sagan discussed the topic in a face-to-face meeting with Pope John Paul II; he also debated it before Congress alongside a critic of the nuclear winter hypothesis, the hydrogen-bomb pioneer Edward Teller. At an antinuclear rally at the U.S. nuclear weapons test site in Nevada in 1986, Sagan was arrested for trespassing. In 1990, Sagan and Turco published A Path Where No Man Thought: Nuclear Winter and the End of the Arms Race, an engrossing review of the history and science behind the nuclear winter thesis. By then, though, the Soviet Union was dis-integrating, and the book sold poorly.

Unwisely, Sagan exploited the nuclear winter thesis as a last-ditch argument against U.S. plans for the Gulf War invasion of Iraq in 1990–1991: He and Turco proposed in Nature that if Iraqi leader Saddam Hussein ignited his nation’s oil wells, the resulting dark clouds might trigger a regional-scale cooling. The Iraqis did start massive oil fires, but the climatic consequences were negligible. Critics charged that in this and other cases, Sagan allowed personal prejudices to distort his scientific judgment.

In a 1994 book, Sagan returned to the optimistic, futuristic—and, thus, commercially more promising— style of his earlier writing, the lavishly illustrated Pale Blue Dot: A Vision of the Human Future in Space (1994). Sagan’s last book published in his lifetime was The Demon-Haunted World (1996). As another retreat from the sophisticated broodings of Shadows, Demon-Haunted World was a series of middlebrow essays about pseudo-science and irrationalism, ideal for the millions of laypersons who enjoyed Sagan’s occasional pieces for Parade, a Sunday supplement inserted in many American newspapers. In an attempt to illustrate his lifelong dedication to scientific method, Sagan listed some of his scientific mistakes (for example, his underestimation of the air density on Venus); yet he neglected to cite the most embarrassing ones (for example, his suggestion that animals as big as polar bears might inhabit Mars).

Sagan’s fame, political activism and occasionally imperious personal manner upset some members of the scientific community. In 1992, when Stanley Miller tried to persuade the National Academy of Sciences to admit Sagan, the subject was vigorously debated; Sagan was rejected. Ironically, two years later, the Academy gave Sagan its Public Welfare Medal in honor of his popularization of science. The Academy citation declared: “No one has ever succeeded in conveying the wonder, excitement and joy of science as widely as Carl Sagan and few as well.”

Sagan suffered for much of his life from achalasia, a swallowing disorder for which he was at times hospitalized. He died from complications of myelodysplasia, a blood disorder, in Seattle, Washington, in December 1996. The scientifically savvy U.S. vice president, Albert Gore Jr., spoke at a funeral service in New York City and cited Sagan’s influence on him: “Those of us who were privileged to bask in that light, however briefly, will never, ever forget its brilliance.”

At present, Sagan’s scientific accomplishments are difficult to assess. He wrote or coauthored more than three hundred scientific articles. His most important research contribution was probably his flawed model for the thermal evolution of the Venusian atmosphere. He taught or inspired a large number of space scientists, and did much to keep popular interest in the National Aeronautical and Space Administration’s space missions alive in the uncertain years after the end of the Apollo Moon missions (1972). Whatever the shortcomings of Sagan and his coworkers’ nuclear winter hypothesis (which, mercifully, was never tested by a full-scale atomic war), he was an inspiring voice for the scientific conscience in an era when many American scientists remained silent while accepting military money to conduct research of debatable merit.

One scholar ranks Sagan among the so-called visible scientists (Goodell, 1977, p. 20) those researchers who popularize their work among the general public. Yet this dry label hardly suggests Sagan’s breadth, complexities, ambitions, and achievements. His showmanship, gaudy surmises, and fascination with intellectually ambiguous figures such as Johannes Kepler and Percival Lowell suggests Sagan belonged to a more discomfiting tradition— what we might call prophetic science or messianic science. (The elderly Arthur Eddington might fall in the same category.) Despite Sagan’s fervent advocacy of “scientific” method, his own methodologies and reasoning at times appeared more reminiscent of Feyerabend’s infamous dictum (1975) that in the history of science, “anything goes.” At the least, Sagan’s colorful career dramatizes a phenomenon emphasized by Kuhn (1962): the disparity between pedagogical idealizations of science and its actual historical practice.

BIBLIOGRAPHY

Sagan’s private papers and a copy of his 265-page résumé are in the possession of his widow, Ann Druyan of Ithaca, New York. An extensive and chronological bibliography of Sagan’s works is in Poundstone (1999).

WORKS BY SAGAN

“Radiation and The Origin Of The Gene,” Evolution 11 (1957):

1, 40–55.

“Physical Studies of Planets,” PhD Diss. University Of Chicago, June 1960.

With Joshua Lederberg. “Microenvironments for Life on Mars.” Proceedings of the National Academy of Sciences 48, no. 9, (1962).

Broca’s Brain: Reflections on the Romance of Science. New York: Random House, 1979.

Cosmos. New York: Random House, 1980.

Contact. New York: Simon & Schuster, 1985.

With Ann Druyan. Comet. New York: Random House, 1985.

With Richard Turco. A Path Where No Man Thought: Nuclear Winter and the End of the Arms Race. New York: Random House, 1990. Although commercially unsuccessful, this book is a treasure trove of information on the history of atmospheric science research relevant to its controversial thesis.

With Ann Druyan. Shadows of Forgotten Ancestors: A Search for Who We Are. New York: Random House, 1992.

Pale Blue Dot: A Vision of the Human Future in Space. New York: Random House, 1994.

The Demon-Haunted World: Science as a Candle in the Dark. New York: Random House, 1996.

Billions and Billions: Thoughts on Life and Death at the Brink of the Millennium. New York: Random House, 1997.

OTHER SOURCES

Broad, William J. “Even in Death, Carl Sagan’s Influence Is Still Cosmic.” New York Times, 1 December 1998.

———. “U.S. Planned Nuclear Blast on the Moon, Physicist Says.” New York Times, 16 May 2000.

Cocconi, Giuseppe, and Philip Morrison. “Searching for Interstellar Communications.” Nature 184 (1959): 844–846. Cooper, Henry S. F., Jr. “A Resonance with Something Alive.” New Yorker, 21 June 1976. The first important biographical essay on Sagan after he became famous. A second installment appeared in the same magazine on 28 June 1976.

Davidson, Keay. Carl Sagan: A Life. New York City: John Wiley & Sons, 1999. A sometimes-critical biography that analyzes Sagan’s life and ideas in their larger historical context.

Sagan, Carl (1934-1996)

World of Earth Science
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Sagan, Carl (1934-1996)

American astronomer

One of the first scientists to take an active interest in the possibility that life exists elsewhere in the Universe, and an astronomer that was both a best-selling author and a popular television figure, Carl Sagan was one of the best-known scientists in the world. Sagan made important contributions to studies of Venus and Mars, and he was extensively involved in planning NASA's Mariner missions. Regular appearances on the Tonight Show with Johnny Carson began a television career that culminated in the series Sagan hosted on public television called Cosmos, seen in sixty countries by over 400 million people. He was also one of the authors of a paper that predicted drastic global cooling after a nuclear war; the concept of "nuclear winter" affected not only the scientific community, but also national and international policy, as well as public opinion about nuclear weapons and the arms race. Although some scientists considered Sagan too speculative and insufficiently committed to detailed scientific inquiry, many recognized his talent for explaining science, and acknowledged the importance of the publicity generated by Sagan's enthusiasm.

Sagan was born in Brooklyn, New York, the son of Samuel Sagan, a Russian emigrant and a cutter in a clothing factory, and Rachel Gruber Sagan. He became fascinated with the stars as a young child, and was an avid reader of science fiction, particularly the novels by Edgar Rice Burroughs about the exploration of Mars. By the age of five, Sagan was sure he wanted to be an astronomer, but, as he told Henry S.F. Cooper, Jr., of the New Yorker, he sadly assumed it was not a paying job; he expected he would have to work at "some job I was temperamentally unsuited for, like door-to-door salesman." When he found out a few years later that astronomers actually got paid, he was ecstatic. "That was a splendid day," he told Cooper.

Sagan's degrees, all of which he earned at the University of Chicago, include an A.B. in 1954, a B.S. in 1955, an M.S. in physics in 1956, and a doctorate in astronomy and astrophysics in 1960. As a graduate student, Sagan was deeply interested in the possibility of life on other planets, a discipline known as exobiology. Although this interest then was considered beyond the realm of responsible scientific investigation, he received important early support from scientists such as Nobel laureates Hermann Joseph Muller and Joshua Lederberg. He also worked with Harold C. Urey, who had won the 1934 Nobel Prize in chemistry and had been Stanley Lloyd Miller's thesis adviser when he conducted his famous experiment on the origin of life . Sagan wrote his doctoral dissertation, "Physical Studies of the Planets," under Gerard Peter Kuiper, one of the few astronomers at that time who was also a planetologist. It was during his graduate student days that Sagan met Lynn Margulis, a biologist, who became his wife in 1957. The couple had two sons before divorcing in 1963.

From graduate school, Sagan moved to the University of California at Berkeley, where he was the Miller residential fellow in astronomy from 1960 to 1962. He then accepted a position at Harvard as an assistant professor from 1962 to 1968. On April 6, Sagan married the painter Linda Salzman; Sagan's second marriage, which ended in a divorce, produced a son. From Harvard, Sagan went to Cornell University, where he was first an associate professor of astronomy at the Center for Radiophysics and Space Research. He was then promoted to professor and associate director at the center, serving in that capacity until 1977, when he became the David Duncan Professor of Astronomy and Space Science.

Sagan's first important contributions to the understanding of Mars and Venus began as insights while he was still a graduate student. Color variations had long been observed on the planet Mars, and some believed these variations indicated the seasonal changes of some form of Martian plant life. Sagan, working at times with James Pollack, postulated that the changing colors were instead caused by Martian dust, shifting through the action of wind storms; this interpretation was confirmed by Mariner 9 in the early 1970s. Sagan also suggested that the surface of Venus was incredibly hot, since

the Venusian atmosphere of carbon dioxide and water vapor held in the sun's heat, thus creating a version of the "greenhouse effect." This theory was also confirmed by an exploring spacecraft, the Soviet probe Venera IV, which transmitted data about the atmosphere of Venus back to Earth in 1967. Sagan also performed experiments based on the work of Stanley Lloyd Miller, studying the production of organic molecules in an artificial atmosphere meant to simulate that of a primitive Earth or contemporary Jupiter. This work eventually earned him a patent for a technique that used gaseous mixtures to produce amino acids.

Sagan first became involved with spaceflight in 1959, when Lederberg suggested he join a committee on the Space Science Board of the National Academy of Sciences. He became increasingly involved with NASA (National Aeronautics and Space Administration) during the 1960s, and participated in many of their most important robotic missions. He developed experiments for the Mariner Venus mission and worked as a designer on the Mariner 9 and the Viking missions to Mars, as well as on the Pioneer 10, the Pioneer 11, and the Voyager spacecrafts. Both the Pioneer and the Voyager spacecrafts have left our solar system carrying plaques which Sagan designed with Frank Drake as messages to any extraterrestrials that find them. The plaques contain pictures of two humans, a man and a woman, as well as various astronomical information. The nude man and woman were drawn by Sagan's second wife, Linda Salzman, and they provoked many letters to Sagan denouncing him for sending "smut" into space. During this project Sagan met the writer Ann Druyan, the project's creative director, who eventually became his wife. Sagan and Druyan had two children.

Sagan continued his involvement in space exploration in the 1980s and 1990s. The expertise he developed in biology and genetics while working with Muller, Lederberg, Urey, and others, is unusual for an astronomer, and he extensively researched the possibility that Saturn's moon , Titan, which has an atmosphere, might also have some form of life. Sagan was also involved in less direct searches for life beyond Earth. He was one of the prime movers behind NASA's establishment of a radio astronomy search program that Sagan called CETI, for Communication with Extra-Terrestrial Intelligence.

A colleague of Sagan's working on the Viking mission explained to Cooper of the New Yorker that this desire to find extraterrestrial life is the focus of all of Sagan's various scientific works. "Sagan desperately wants to find life someplace, anyplace—on Mars, on Titan, in the solar system or outside it. I don't know why, but if you read his papers or listen to his speeches, even though they are on a wide variety of seemingly unrelated topics, there is always the question 'Is this or that phenomenon related to life?' People say, 'What a varied career he has had,' but everything he has done has had this one underlying purpose." When Cooper asked Sagan why this was so, the scientist had a ready answer. "I think it's because human beings love to be alive, and we have an emotional resonance with something else alive, rather than with a molybdenum atom."

During the early 1970s Sagan began to make a number of brief appearances on television talk shows and news programs; Johnny Carson invited him on the Tonight Show for the first time in 1972, and Sagan soon was almost a regular there, returning to discuss science two or three times a year. However, it was Cosmos, which Public Television began broadcasting in 1980, that made him into a media sensation. Sagan narrated the series, which he wrote with Ann Druyan and Steven Soter, and they used special effects to illustrate a wide range of astronomical phenomena such as black holes. In addition to being extremely popular, the series was widely praised both for its showmanship and its content, although some reviewers had reservations about Sagan's speculations as well as his tendency to claim as fact what most scientists considered only hypotheses.

Sagan was actively involved in politics; as a graduate student, he was arrested in Wisconsin for soliciting funds for the Democratic Party, and he was also involved in protests against the Vietnam War. In December, 1983, he published, with Richard Turco, Brian Toon, Thomas Ackerman, and James Pollack, an article discussing the possible consequences of nuclear war. They proposed that even a limited number of nuclear explosions could drastically change the world's climate by starting thousands of intense fires that would throw hundreds of thousands of tons of smoke and ash into the atmosphere, lowering the average temperature ten to twenty degrees and bringing on what they called a "nuclear winter." The authors happened upon this insight accidentally a few years earlier, while they were observing how dust storms on the planet Mars cooled the Martian surface and heated up the atmosphere. Their warning provoked a storm of controversy at first; their article was then followed by a number of studies on the effects of war and other human interventions on the world's climate. Sagan and his colleagues stressed that their predictions were only preliminary and based on certain assumptions about nuclear weapons and large-scale fires, and that their computations had been done on complex computer models of the imperfectly understood atmospheric system. However, despite numerous attempts to minimize the concept of a nuclear winter , the possibility that even a limited nuclear war might well lead to catastrophic environmental changes was supported by later research.

The idea of nuclear winter not only led to the reconsideration of the implications of nuclear war by many countries, institutions, and individuals, but it also produced great advances in research on Earth's atmosphere. In 1991, when the oil fields in Kuwait were burning after the Persian Gulf War, Sagan and others made a similar prediction about the effect the smoke from these fires would have on the climate. Based on the nuclear winter hypothesis and the recorded effects of certain volcanic eruptions , these predictions turned out to be inaccurate, although the smoke from the oil fires represented about 1% of the volume of smoke that would be created by a full-scale nuclear war.

In 1994, Sagan was diagnosed with myelodysplasia, a serious bone-marrow disease. Despite his illness, Sagan kept working on his numerous projects. His last book, The Demon-Haunted World: Science as a Candle in the Dark, was published in 1995. At the time of his death, Sagan was co-producing a film version of his novel Contact. His partner in this project was his wife, Ann Druyan, who had co-authored Comet. Released in 1997, the film received popular and critical acclaim as a testimony to Sagan's enthusiasm for the search for extraterrestrial life. Sagan, who lived in Ithaca, New York, died at the Fred Hutchinson Cancer Research Center in Seattle in 1996.

Carl Sagan won a Pulitzer Prize in 1978 for his book on evolution called The Dragons of Eden. He also won the A. Calvert Smith Prize (1964), NASA's Apollo Achievement Award (1969), NASA's Exceptional Scientific Achievement Medal (1972), NASA's Medal for Distinguished Public Service (twice), the International Astronaut Prize (1973), the John W. Campbell Memorial Award (1974), the Joseph Priestly Award (1975), the Newcomb Cleveland Prize (1977), the Rittenhouse Medal (1980), the Ralph Coats Roe Medal from the American Society of Mechanical Engineers (1981), the Tsiolkovsky Medal of the Soviet Cosmonautics Federation (1987), the Kennan Peace Award from SANE/Freeze (1988), the Oersted Medal of the American Association of Physics Teachers (1990), the UCLA Medal (1991), and the Mazursky Award from the American Astronomical Association (1991). Sagan was a fellow of the American Association for the Advancement of Science, the American Academy of Arts and Sciences, the American Institute for Aeronautics and Astronautics, and the American Geophysical Union. Sagan was also the chairman of the Division for Planetary Sciences of the American Astronomical Society (from 1975 to 1976) and for twelve years was editor-in-chief of Icarus, a journal of planetary studies.

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Carl E. Sagan

Encyclopedia of World Biography
COPYRIGHT 2004 The Gale Group Inc.

Carl E. Sagan

The American astronomer and popularizer of science Carl E. Sagan (1934-1996) studied the surfaces and atmospheres of the major planets, conducted experiments on the origins of life on earth, made important contributions to the debate over the environmental consequences of nuclear war, and wrote a number of popular books explaining developments in astronomy, biology, and psychology.

Carl Edward Sagan was born November 9, 1934, in New York City. Pursuing a boyhood fascination with the stars, he studied astronomy at the University of Chicago, receiving his undergraduate degree in 1954 and his doctorate in 1960. After holding teaching and/or research posts at the University of California-Berkeley, Harvard University, the Smithsonian Astrophysical Observatory, and Stanford University, Sagan became director of Cornell University's Laboratory for Planetary Studies and David Duncan Professor of Astronomy and Space Science (1970). In addition to his academic appointments Sagan served as a consultant to the National Aeronautics and Space Administration (NASA) and was closely associated with the unmanned space missions to Venus, Mars, Jupiter, and Saturn. Sagan's work in the popularization of science, which brought him public recognition as author, lecturer, and television personality, won for him the Pulitzer Prize in 1978.

Carl Sagan's main contributions to science were made in the fields of planetary studies and the origin of life. His first major research effort was an investigation of the surface and atmosphere of Venus. In the late 1950s the prevailing scientific view was that the surface of Venus was relatively cool, life of some sort might exist on the planet, and the observed Venusian radio emissions had their origins in the activity of charged particles located in an atmospheric layer. Sagan (1961) overturned this by showing that the emissions could be explained by simply assuming that the Venusian surface was very hot, over 300 degrees Centigrade, and therefore hostile to life. He accounted for the high temperatures by positing the existence of a "greenhouse effect" that resulted from the sun's heat being trapped between the Venusian surface and the planet's carbon dioxide cloud cover. This hypothesis was confirmed by an exploratory space vehicle sent to Venus by the Soviet Union in 1967.

Solar System Research

The physical characteristics of the surface of Mars have long interested astronomers and science fiction writers. Telescopic
observation of the planet revealed distinctive bright and dark areas on its surface. This led some to speculate that large regions of Mars were covered with vegetation subject to seasonal changes. So matters stood until the mid-20th century, when radar and other new means of surveillance were used to gather information on the topography, temperature, wind velocities, and atmosphere of Mars. Reviewing this newly collected data, Sagan concluded that the bright regions were lowlands filled with sand and dust blown by the wind and that the dark areas were elevated ridges or highlands. Hence there was no need to assume the existence of extensive Martian plant growth.

Sagan's scientific interest in planetary surfaces and atmospheres led him to investigate the origins of life on earth and to champion the study of exobiology (the biology of extraterrestrial life). In the mid-1950s Harold Urey and Stanley Miller successfully produced key organic compounds in the laboratory by simulating the physical and chemical conditions that prevailed upon earth shortly before the first forms of life appeared. Building upon this research, Sagan irradiated a mixture of methane, ammonia, water, and hydrogen sulfide. In these experiments he was able to produce amino acids and adenosine triphosphate (ATP), complex chemical compounds which are crucial to living cells. Sagan's foray into biochemistry contributed to a better understanding of the nature and origins of terrestrial life and testified to his competence in fields of science beyond astronomy.

Popular Writing

It is not his scientific achievements but his popular books and his television appearances that have made Sagan a well-known public figure. In 1973 he published The Cosmic Connection, a lively introduction to space exploration and the search for extraterrestrial life. Four years later there appeared his Pulitzer Prize winning book on the evolution of human intelligence, The Dragons of Eden. Drawing upon recent work in neuro-physiology and exploring the brain-computer analogy and studies of sleep and dreaming, as well as interpretations of mythology, Sagan created a highly readable, original, and witty account of the development of the human intellect. Another of Sagan's books, Cosmos (1980), deserves notice because it was written in conjunction with his well-received television series of the same name. In this work Sagan offered a summary history of the physical universe, showed how the cosmos came to be understood with the help of modern science, and warned that the earth was in danger of being destroyed by a nuclear holocaust.

In December of 1983 Sagan, with colleagues R. P. Turco, O. B. Toon, T. P. Ackerman, and J. B. Pollack, published "Nuclear Winter: Global Consequences of Multiple Nuclear Explosions," an article which transformed the world-wide public debate over nuclear policy. The authors claimed that in a nuclear war tremendous quantities of soot and dust would be injected into the atmosphere to form a gigantic black cloud covering most of the Northern Hemisphere. This cloud would reduce the incoming sunlight by more than 95 percent for a period of several weeks and affect the climate on earth for a number of years thereafter. During the cold, dark nuclear winter the vegetation which animals and humans need for sustenance would be seriously depleted and great harm would be done to the ecosystem and to human society. In this instance, as so often during the course of his career, Sagan drew upon his extensive knowledge of the forces operating in the atmosphere. However, that knowledge was not used to explain the features of some remote planet but to send a message warning the entire human race of the terrible consequences of nuclear warfare.

Sagan continued to work and proselytize for the furthurance of science until his death in Seattle on December 20, 1996, of pneumonia brought on by a rare bone marrow disease. In July of the following year, upon successful touchdown and deployment on the surface of Mars, the Pathfinder Lander was renamed The Dr. Carl Sagan Memorial Station.

Further Reading

Sagan's career in science and public life to the mid-1970s is covered in Henry S. F. Cooper, Jr., "Profiles (Carl Sagan—I, II)," The New Yorker, June 21 and 28, 1976. Among Sagan's many publications see: Communication with Extraterrestrial Intelligence (1973), The Cosmic Connection (1973), The Dragons of Eden (1977), Broca's Brain (1979), Cosmos (1980), Comet (1985), and Contact (1985).

Additional Sources

Rae Goodell, The Visible Scientist, Little, Brown, 1975.

Carl Sagan, and Richard Turco, A Path Where No Man Thought: Nuclear Winter and the End of the Arms Race, Random House, 1990.[/bibcit.composed

Carl Sagan

The American astronomer Carl Sagan studied the surfaces and atmospheres of the major planets, conducted experiments on the origins of life on Earth, made important contributions to the debate over the environmental consequences
of nuclear war, and wrote a number of popular books explaining developments in astronomy, biology, and psychology.

Younger years and education

Carl Edward Sagan was born on November 9, 1934, in New York City to Samuel and Molly Sagan. His father was a factory manager. As a child Carl was fascinated with the stars. By age nine he was an amateur astronomer and felt certain there was life on other planets. At age twelve he announced to his grandfather his intention to become an astronomer. However, it was not until he was a sophomore in high school that he realized that astronomers actually got paid for their work.

The family later moved to Rahway, New Jersey, where Carl graduated from high school in 1951. He was voted "most likely to succeed" and "most outstanding male student." With the help of several scholarships, he studied astronomy at the University of Chicago. He was captain of a championship intramural basketball team and president of the Astronomical Society, which he had founded. He received a bachelor of arts degree with special honors in the natural sciences in 1954, a bachelor of science degree in physics in 1955, and his doctorate in 1960.

Early teaching and research

Over the next ten years Sagan held teaching and/or research posts at the University of California at Berkeley, Harvard University, the Smithsonian Astrophysical Observatory, and Stanford University. In 1970 he became director of Cornell University's Laboratory for Planetary Studies and David Duncan Professor of Astronomy and Space Science. In addition to his academic appointments Sagan served as a consultant to the National Aeronautics and Space Administration (NASA) and was closely associated with the unmanned (without astronauts onboard) space missions to Venus, Mars, Jupiter, and Saturn.

The surface and atmosphere of Venus

Sagan's first major research effort was an investigation of the surface and atmosphere of Venus. In the late 1950s the general scientific view was that the surface of Venus was relatively cool and that life of some sort might exist on the planet. Radio emissions had been
observed and it was thought that they came from the activity of charged particles located in an atmospheric layer.

Sagan overturned these ideas in 1961 by showing that the emissions could be caused by a very hot surface temperature, over 300 degrees Centigrade (572 degrees Fahrenheit), in which life could not exist. He said the high temperatures were caused by a "greenhouse effect," in which the sun's heat was trapped between the planet's surface and its carbon dioxide cloud cover. These ideas were confirmed by an exploratory space vehicle sent to Venus by the Soviet Union in 1967.

Solar system research

The physical characteristics of the surface of Mars have long interested astronomers and science fiction writers. Telescopic observation of the planet revealed distinctive bright and dark areas on its surface. This led some to guess that large regions of Mars were covered with plants that change with the seasons.

In the mid-twentieth century, radar and other new means of observation were used to gather information on the topography (surface features), temperature, wind speeds, and atmosphere of Mars. Reviewing this newly collected data, Sagan concluded that the bright regions were lowlands filled with sand and dust blown by the wind and that the dark areas were elevated ridges or highlands.

The origins of life

Sagan also investigated the origins of life on Earth and championed the study of exobiology (the biology of extraterrestrial life). In the mid-1950s Harold Urey and Stanley Miller had successfully produced key organic compounds in the laboratory by recreating the physical and chemical conditions that were common on Earth shortly before the first forms of life appeared. Building upon this research, Sagan exposed a mixture of methane, ammonia, water, and hydrogen sulfide to radiation. This had the effect of producing amino acids and adenosine triphosphate (ATP), complex chemical compounds that are crucial to living cells.

Popular writing

It is not his scientific achievements but his popular books and his television appearances that made Sagan a well-known public figure. In 1973 he published The Cosmic Connection, a lively introduction to space exploration and the search for extraterrestrial life. Four years later he published his Pulitzer Prize-winning book on the evolution of the human brain, The Dragons of Eden.

Another of Sagan's books, Cosmos (1980), deserves notice because it was written in conjunction with his well-received television series of the same name. In this work Sagan offered a brief history of the physical universe, showed how the universe came to be understood with the help of modern science, and warned that the Earth was in danger of being destroyed by a nuclear disaster.

Nuclear winter

In December of 1983 Sagan, with colleagues R. P. Turco, O. B. Toon, T. P. Ackerman, and J. B. Pollack, published "Nuclear Winter: Global Consequences of Multiple Nuclear Explosions." The article stated that in a nuclear war tremendous quantities of soot and dust would be injected into the atmosphere to form a gigantic black cloud
covering most of the Northern Hemisphere. This cloud would reduce the incoming sunlight by more than 95 percent for a period of several weeks and affect the climate on Earth for a number of years thereafter. During the cold, dark nuclear winter the vegetation which animals and humans need for sustenance (nourishment) would be seriously depleted and great harm would be done to the environment and to human society.

Sagan continued to work and speak about science until his death in Seattle, Washington, on December 20, 1996, of pneumonia brought on by a rare bone marrow disease.

Sagan once said that his own sense of wonder, combined with the encouragement his parents showed for pursuing unusual interests, were the factors that led him to his choice of work. He was best known for popularizing science, presenting it in generally understandable or interesting form. It is a fitting tribute that the Mars Pathfinder lander was renamed the Dr. Carl Sagan Memorial Station when it touched down on the surface of Mars in July of 1997.

Sagan, Carl

Space Sciences
COPYRIGHT 2002 The Gale Group Inc.

Sagan, Carl

American Astronomer, Author, and Educator 1934-1996

Carl Sagan was a Pulitzer-prize winning author, visionary educator, and devoted scientist. He worked to extend humankind's reach into the solar system, and to help people understand the importance and meaning of the scientific method. Born on November 9, 1934, Sagan conducted his undergraduate work at Harvard University, and earned doctorates in astronomy and astrophysics at the University of Chicago. He was named a professor at Cornell University in Ithaca, New York, in 1971. Sagan's academic research concentrated on biology, evolution, astrophysics, planetary science, and anthropology.

Sagan was the author of more than 600 academic papers, twenty books, and a television miniseries called Cosmos. His novel about contact with an extraterrestrial civilization, Contact, was made into a popular Hollywood film in 1997. Much of Sagan's life was devoted to debunking scientific
misconceptions and advocating clear thinking and better appreciation for the basics of science and its importance in everyday life. He was also a strong advocate for space exploration, especially robotic exploration of the solar system and beyond. Sagan also supported nuclear disarmament and urged the United States and the then-Soviet Union to undertake a joint mission to explore Mars.

Sagan was cofounder of the Planetary Society, a nonprofit organization supporting the exploration of space. He died on December 20, 1996, in Seattle, Washington, at the age of sixty-two.

see also Astronomy, Kinds of (volume 2); Literature (volume 1).

FrankSietzen, Jr.

Bibliography

Poundstone, William. Carl Sagan: A Life in the Cosmos. New York: Henry Holt, 2000.

Sagan, Carl Edward

The Columbia Encyclopedia, 6th ed.

Copyright The Columbia University Press

Carl Edward Sagan (sā´gən), 1934–96, American astronomer and popularizer of science, b. New York City. Early in his career he investigated radio emissions from Venus and concluded that the cause was a surface temperature of c.900°F (500°C) and crushing atmospheric pressure. He also studied color variations on Mars' surface, concluding that they were not seasonal changes as most believed but shifts in surface dust caused by windstorms. Both conclusions were substantially confirmed years later by space probes. Sagan is best known, however, for his research on the possibilities of extraterrestrial life (see exobiology), including experimental demonstration of the production of amino acids from basic chemicals by radiation. A professor of astronomy and space sciences at Cornell after 1968, he was involved with numerous NASA planetary space probes and was the creator and host of the 1980 public television science series Cosmos. His publications include The Dragons of Eden (1977; Pulitzer); a novel, Contact (1985); with Richard Turco, A Path Where No Man Thought (1990), on nuclear winter; with Ann Druyan, Shadows of Forgotten Ancestors (1992); Pale Blue Dot (1994); and The Demon-Haunted World (1995).

See biographies by K. Davidson (1999) and W. Poundstone (1999).

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